1. Field of the Invention
The present invention relates to a developing unit that supplies toner to an electrostatic latent Image formed on a surface of an image bearing body, and more particularly to a developing unit that has a developing toiler and a supplying roller and a toner accommodating section defined above the developing roller and supplying roller.
2. Description of the Related Art
A conventional electrophotographic color printer incorporates a plurality of image forming sections. Each of the image forming sections includes a developing unit to which a toner cartridge is detachably mounted.
The image forming section incorporates a photoconductor, a charging roller, a developing unit, and a cleaning section. The charging roller charges the surface of the photoconductor. An electrostatic latent image is formed on the charged surface of the photoconductor in accordance with print data. The developing unit causes toner to be deposited on the electrostatic latent image to develop the electrostatic latent image into a toner image. The developing unit includes a developing roller, a developing blade, and a supplying roller. The developing roller rotates in contact with the photoconductor to deposit toner on the surface of the photoconductor. The developing blade is in contact with the developing roller to regulate the thickness of the toner layer formed on the developing roller. The toner supplying roller rotates in contact with the developing roller to supply toner to the developing roller. A toner cartridge and a toner accommodating section are disposed away from the photoconductor.
A tandem type electrophotographic printer has a plurality of image forming sections spaced apart by an equal distance and aligned straight. The image forming sections are disposed over a belt on which a print medium is transported from section to section in sequence. The print medium passes under the respective image forming sections where toner images are transferred onto the print medium in order.
The problem with a tandem type printer is that the printer has a long dimension in a direction of travel of the print medium. In addition, with the advance of office automation in recent years, printing is performed more frequently. Thus, toner in a printer is used up soon and a need exists for a large toner accommodating space that requires less frequent replacement of the toner cartridge. However, providing a large toner accommodating space leads to a printer of a large size. This is against the users"" trend toward a small size printer.
Attempts have been made to solve the aforementioned drawbacks of the conventional developing unit. That is, a larger toner accommodating section and a larger toner cartridge are disposed over a portion at which the developing roller is in contact with the supply roller, while also making the overall dimension of the printer shorter in a direction in which a plurality of image-forming sections are aligned.
The aforementioned conventional printer has a toner accommodating section and a toner cartridge disposed above an area at which the developing roller and supply roller are in contact with each other. The toner supplied from the toner cartridge causes a decrease in the fluidity of toner near the area in which the developing roller and supply roller are in contact with each other. Thus, old, clumped toner or foreign materials in the toner are trapped between the developing blade and the developing roller, preventing the fresh, new toner from being supplied thereto.
Foreign materials trapped between the developing blade and the developing roller cause white narrow areas or lines in a printed image in which toner is absent. Toner clumped between the developing blade and the developing roller causes partial insufficient toner deposition in the printed image.
FIG. 8 illustrates an example of print results in which when printing is performed with a print medium running in a direction shown by arrow A, some areas in the printed image have insufficient toner deposited. FIG. 8 shows a black image solidly printed over the entire area of print medium. It is to be noted that regions of insufficient deposition of toner P appear irregularly on a print medium 20 while other areas Q are printed black.
An object of the present invention is to provide a developing unit with fluid toner in an area where a developing roller and supplying roller are in contact engagement with each other, thereby preventing foreign materials or clumped toner from being trapped in the area.
Another object of the invention is to prevent white lines or low-density areas from appearing in a printed image.
A developing unit is disposed substantially directly under a toner chamber and receives toner through an opening formed in the toner chamber, and includes a developing roller, a supplying roller, and a developing blade. The developing roller supplies toner to an electrostatic latent image formed on an image bearing body. The supplying roller rotates in contact with the developing roller to supply the toner to the developing roller. The developing blade is in pressure contact with the developing roller to form a thin layer of toner on the developing roller. The developing blade is in contact with the developing roller at a first position spaced at least 5.84 mm from a second position at which the supplying roller is in contact with the developing roller. An agitating member may be added over the developing, roller and supplying roller and between the first position and the second position.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.